Composite pack for electrical heating elements



Patented Mar. 17, 1925.

' UNITED-[STATES I WILLIAI A. name, or

PATENT OFFITCEV.

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To all whom it may concern:

Be it known that I,'WILIJAM A. BRAUN, a citizen of the United States of America, and resident of Dover, in the county of l Tuscarawas and State of Ohio, have invented certain new and useful Improvements ir Composite Packs for Electrical Heating Elements, of which the following is a specification, 1 f

This invention relates to improvements in compositions for making packs for heating elements in which electrical resistance wires are embedded and insulated thereby from the heated members. I Ordinarily, electricalheating elements are made by embedding aresistance wire in av pack composed of cement which is molded in the member to be heated and then subjecting the molded pack to enormous pres- 2 sure and exceedingly .high temperature in order to enhance its di-electric'and heat conducting properties.- Such packs are ordinarilyformed with a base of pulverulent' refractory material and a bonding 2 clay, which composition after being highly compressed and heated tends to become brittle and porous.

The objects oft-he invention are to provide a composition by meansof which packs no of great; density may be formed and which will be practically im ervious to moisture and afford permanent msulationfor the. re-

sistance wire embedded therein-and be possessed of high thermal conductivity These objects are accomplished by the i use of the (group of materials compounded and applie as hereinafter set forth. The invention includes a composite substance adapted for making packs for elec- 40 trical heating elementsfi formed into a dried pack from a plastic mass of cement constituted of a mlxture of refractory material, clay and boracic acid moistened with waten The components of the cement are in relaac tive roportions as set forth in the following tabu ation:

Per cent. Refractory material 83 Clay 12 Boric acid 5 The refractory material employed is such as alundum fines commonly known in the Application men January 5, 1823. Serial No. 810,944.

art or may be such other hard, granular or pulverulent material of similar character which isdpossessed of high thermal conductivity an high di-electric properties.

The clay herein used as an agent for holding the pulverulent refractory material suspended in the mixture when the mixture is moistened and in a plastic state, may be of any suitable character, such, for example, as Klingenberg clay, which has been found satisfactory.

The boracic acid, when introduced into the mixture, is in-powdered form.

In preparing the cement, the ingredients are placed in a pebble mill with water approximately equal in quantity to half of the amount of powder, by weight, and therein is ground and worked into a plastic mass. The compositionthus formed is then placed in a suitable mixing machine and agitated until used.

The'plastic cement is molded or formed into a pack in a manner to embed the resistance wire of the heating element so that the resistance wire is completely encased by the pack. After thus being formed the pack is dried until it becomes hard and is thenv raised to thenorm'al working temperature of the heating element in which it is used. This heating'operation may be carried out by charging the resistance wire initiall with electric current of suificient stren to heat the resistance wire to its workmgtem rature. When the pack is thus heated t e boracic acid constituent of the mixture of which the pack is composed,

becomes melted, forming boric oxide (B 0 which saturates the pack during the initial heating thereof so that the granular particles of the refractory constituent thereof, and the resistance wire encased therein become coated therewith, and the interstices between the.particles and about the wire become filled. Thus, the pack is rendered exceedingly dense and its porosity is correspondingly reduced and its di electric pro erty consequently is preserved. Also, t e intimate coating thus formed'upon the resistance member prevents its oxidation so that its life is indefinitely protracted.

The constituents of the mixture and their relative pro ortions, set forth herein as an example, of the invention, are subject to such substitutions and variations of proportions as skill in the art afiords, and there acid, the grains of said refractory material being coated with'said acid.

2. A pack;for electrical heating elements composed of a conglomerate 'non yitrified mass of grains of refractory material and clay, and an inter-mixture of boraclc ac1d forming a coatingon said grains and filling the interstices therebetween. 15

In testimony whereof I aflix. my signature in presence of two Witnesses.

, WILLIAM A. BRAUN. I -Witnesses: v I

CHAS. Wmmnn, ZOE A1. MUCKLEY. f 

